Process for conversion of so2 to so3



" June l1, 1929. f J.. G. MELENDY 1,716,498`

rPROCESS FOR CONVERSION OF' SO2 T0 S03 Filed Jan. 26, 192'? 8 i 60 l i 70 /o 20 .sa 4a 5a 60 7a a@ @a fa@ //0 /za @a /40/5@ /60 Ama/Vr of @4720/57 F/ Z INVENTR JESSE a MELE/wy BY M j@ ATTORNE stance.

Patented June 11, 1929.

Utu'rlazn4 STATES PATENT OFFICE.

JESSE G. -MELENDY, OF TARRYTOWN, NEW YORK, ASSIGNOR TO GENERAL CHEMICAL COMPANY, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

PROCESS FOR CONVERSION OF SO2 T0 S03.

Application led January 26, 19.27. Serial No. 163,595.

This invention relates to an .improved method of conducting the conversion 4of sul-v fur dioxide to sulfur trioxide by means of Contact or catalytic processes.

The object of the invention is the provision of an improved method wherein optimum operating conditions may be closely approached in commercial practlee and considerable saving effected in the amount of platinum or 4other catalyst required tobrmg about. a substantially complete conversion of sulfur dioxide to trioxide.

The reaction of sulfur dioxide plus oxygen to give-sulfur trioxide is an exothermal reversible reaction, as is well-known, and considerable heat is evolved as the reaction taires place. When operating in accordance with the contact process this reaction is caused to take place in the presence `of a catalyst, to

- increase the rate of reaction. y Of those catalysts known at the'presenttime, platinum is the mostsatisfactory and most widely used. The heat generated by the conversion of the sulfur dioxide'to sulfur trioxide when not I continuously removed raises the temperature of the gases tosuch an extent that an equilibrium condition is approached. As the conditions of temperature and conversion approach the point of equilibrium, the rate of reaction falls oif to an exceedingly small value. To carry on the conversion under such conditions requires a very long time of contact of the gases with the catalyst or in other terms, a very large amount of contact sub.

On the other hand, when the temperature is favorable and the conditions of con-version and temperature are lmaintained considerablv below equilibrium the rate of reaction is high so that only a short time of contact of the gases with the Catalyst or a smaller arnount'of catalyst is, required. As

the cost of the catalyst, particularly platinum, is a major item oiexpense inthe production of sulfuric acid by the costaet process, a reduction the amount of catalyst required is of importance. xplleretofore various methods have been pro- -thc'necessity for temperaturev control during the conversion process and has endeavored to provide a method of obtaining quantitative 55 conversion by removing the excess heat of the reaction from the contact chamber. He has also pointed out in his U. S. Patent #823,472, the desirability of conducting the conversion process in a plurality of stages with intermediate cooling of the partially converted gases. The systems thus fardisclosed are, however, open to the objections that definite, controlled temperature conditions cannot be maintained in the systems and that a proper distribution of the catalyst has notbeen attained. I have discovered certain further essential operating conditions and improvements :in the general process whereby results approximating those obtained under theoretical optimumoperating conditions may be obtained.

v- The conditions which I have found to be necessary for most eilicient operation are as follows:

1. 'Maintenance of uniform temperature conditions throughout the catalyst at right angles to the direction o'f gas How.

2. Positive, controlled regulation of the y temperature of the gases entering each conversion stage. l 1

3. Correct proportioning oftheamount of catalyst in the .sev-eral conversion stages. as more yfully explained hereinafter. After considerable experimentation I'-have found that all of theabove necessary conditions'are maintained when operating as follows;

1. Carrying on the conversion reaction in adiabatic stages.

2. Cooling the gases with positive, control lable cooling means intermediate the conversion stages.

3. So proportioning the amount of catalyst 1n each stage except the last stage, as to right angles to the direction of the catalyst chamber 30 within a Contact mass external cooling.

fur dioxide,

cause the gas to pass out of contact with the catalyst, and thus stop the reaction when 'he rate of-the reaction begins to fall oil, anc so proportioning the amount of catalyst in the 5 last stage as to bring the reaction to substantial equilibrium vonly in this stage.

By carrying on the reaction in.adiabatic `stages, I am enabled to maint-ain uniform temperature conditions in the catalyst at gas flow and thus am able to determine temperature conditions throughout the entire mass ofthe catalyst. This of'course is not possible with those lsystems in which positive cooling of is employed, and a highly Variable heat gradient obtained from the center of the contact mass outwardly. It is practically impossible to determine what temperature is existing at any given point Where there is positive external cooling and the teinpeiatuie varies not only in the direction of gasilow but also at right angles to this direction. `Furthermore, it is impossible to predetermine what 25 temperature changes will take place if the gas speed, or strength of gas, is changed, as not only is the h'eat generated changed, but likewise the cooling effect. Contrasted with this .is the operation of'a converter Without By not permitting any ,substantial heat gradient to exist at right angles tothe direction of 4gas How, and only having to contend with a rection of gas flow,'it becomes 'possible to predetermine conditions, because the only substantial change brought-about by a. change in gas speed, or strength of gas, is a change in i the heat generated. It is only by accurate knowledge of temperature conditions, such as o 'is obtained in accordance with my procedure,

that etlicient operation becomes possible.-

The provision of positive controllable cooling means intermediate the conversion stages ermits any desiredl definite temperature to e' maintained in .the gases enteringeach conversion stage. As above pointed out, the temperature k reached in any conversion stage will depend upon the-amount of sulfur dioxide converted .temperature rises the rate of the reaction decreases.- This rate is high and relatively constant up to a certain temperature, which may be determined for each concentration of suland then above this. temperature begins to fall ol very rapidly. By providing o'nly sutlicient platinum toreach the point Where vthe rate of conversion' begins to fall off, I am enabled to convert a much larger quanto sulfurn trioxide in this stage, and as tliis ;0 tity of sulfur dioxide to sulfur trioxide with a given amount of platinum.

For purposes of illustration I have shown lily-invention inthe accompanying drawing as applied to a three-stage converter system.

i5 The method of operation of the known multivariation in the di? `a point B in Fig.

stage converter system together with my method, are diagrammatically shown together in order that the distinction between my process and the present art may bel easily production is equivalent to the amountof catalyst required to effect the said conversion. 'Ihe curves ot Fig. 2 represent'spccilically the -course o'l the conversionreaction when using platinum as the catalyst, although the general form of such curves will be similar irrespective of the particular catalyst.

The common method of stage converter with intermediate cooling' would be illustrated by leading the gases from a sulfur burner 1 4through the cooler 2 before introducing said gases into the first converter 3. In this converterfthe conversion is carried up to the point A on the curve of Fig. 2, the point A representing practically equilibrium conditions, or at least substantially tlie'maximum conversion possible of being obtained in operating a iiiultii a single converter of whatever type employed. e

The gases Would then be cooled inicooler 4 before passing into the next converter 5, wherein the saine operation as in converter 3 Would be repeated and 2. The gases would then be cooled again in the cooler 6 and passed to the inal converter 7, wherein the conversion would be carried up to the point C in Fig. 2.

According to the present method, a sulfur dioxide gas mixture is evolved from burner 1 'and cooled in cooler`2 to the temperature dcsired for the` first conversion stage, which will be in the neighborhood of 400 2C. or that temperature most eflicient for the gas mixture used. This gas mixture at a definite temperature is introduced into converter 3. 'F roma study of Fig. 2 representing the course of the conversion reaction, it will be seen that the rate of conversion 1s quite rapid up to Within -the neighborhood of the the conversion carried up to A', the gases then being withdrawn from the converter and-subjected to ya cooling action in the cooler 4. I

The gases, after beirg cooled in cooler 4 to substantially 400 G. are then introduced into converter 5, wherein conversion takes place along the curve A B of Fig. 2. Following the principle employed by me, only ythat amount of platinum is placed in converter 5 as will carry the reaction to the point B or at that point where the rate of reaction again begins to fall off.

The gases are then cooled in cooler 6 and introduced into the final converter 7 in which the conversion is carried to whatever overall conversion is desired in the system, which ordinarily'will be in the neighborhood of fk5-97%.

In the final converter 7 only is placed sufiicient platinum to bring the reaction to substantial equilibrium. Fig. 2, the amount of platinum required to carry the reaction from point B to point C is much greater than that required to reach point A or point B. This is due t-o the fact that the amount of platinum required to overcome the low reaction rate obtaining at the higher percentages of conversion when approaching equilibrium is much greater than when operating at the. higherreaction rates obtaining at the lower conversions when operating in ac` cordance with my process. Following the conversion in converter'7 the gases are then cooled in cooler 8 and passed to the absorber 9 in which the sulfur trioxide is absorbed in sulfuric acid to produce the final product.

" By comparing the relative amounts of platinum required to reach the points C and C in Fig. 2, which points represent substantially the same overall conversions, it will be seen that my procedure enables Ithe quantity of platinum to be very materially reduced. This is because of the great saving in platv inum which I obtain in the earlier stages of conversion by stopping the reaction in these stages as soon as the reaction rate begins to fall ofi".

.When employing a gas containing 842% SO2 and conducting the conversion in three stages I propose to stop the conversion in the first stage when it has reached a value-of y842% below that which might be obtained at' equilibrium if the reaction were continued adiabatically. The conversion in the second stage is carried to a value about 4-6% below the corresponding equilibrium value, while the conversion is caused to go to substantial equilibrium in the third or last converter. Thus where gas containing 8-12% SO2 gas is introduced into the successive converters at approximately 400 C. the equilibrium conversions in the first and second converters will be about and 90% respectively. I have found that most efficient operation is obtained, however, if the conversion be carried only to 60% in the first converter, 85%' in the second converter and completed to S35-97% in the last converter. When operating in this manner, I have found that for a three-stage As will be seen fromconverter system operating on an 842% gas the proportions of platinum in the various stages should be in the ratio of 'fl-6% in the' first stage, 104404; in the second stage and -86% in the third stage.

Then the conversion is carried out in a system with such a distribution ot' platinum and inpadiabatio stages with positive intermediate cooling, temperature conditions will be obtained which approach the theoretical optimum conditions for obtaining final conversions of -9770 and thus require a minimum quantity of platinum.

It will be seen that by stopping the reaction' ers is much greater than the additional amount required in the final converter to handle the slightly greater amo uut of sulfur dioxidepresent here than in the final converter of the known systems. The overall saving is considerable and results in lower operating costs.

In applying my invention to commercial practice in the design of the converter units of a contact sulfuric acid plant, or in repacking the converter .units of an. operating plant to obtain greater efiiciency, the basist'or calculations will be the volume and the percentage strength of sulfur dioxide -gas which must be handled per unit of time to give the tonnage of sulphuric acid required of the plant. The percentage strength of sulfur dioxidevwill be determined b'y the nature of the sulfur-bearing raw material (pyrites ore, brimstone, etc.) and the type of ore o'r brimstone burning equipment. The volume of this strength of gas which-must be handled to produce the tonnage of acid required will'enable the designing engineer -to determine the dimensions of the contact chambers'in which the catalyst is to be placed to give commercial gas speeds throughthe system, in accordance .with standard practice in the contact sulfuric.

acid art. A satisfactoryy gas speedhaving been determlned upon, the next consideration will be the amounts of catalyst to be placedl 1n the various sta-ges of the system to give the l proper time of contact ofxthe gas with the; catalyst in each stage.

' VThe amount of catalyst will depend upon ,the gas speedand strength of sulfur dioxide (previously determinedas explained above) and the nature of the catalystwhi'ch is to be used. Factors affecting the nature of the catalyst are the chemical-and physical properties of the catalyst, particularly the activity ofthe.` catalyst, Athe type of catalyst carrier (asbestos, magnesium, sulfate, etc.) the meth.

od of distribution of the catalyst upon the carrier, the concentration of catalyst in the carrier, the method of packing the catalyst in the contact chamber, etc., and consequently the nature and activity ofdiife'rent catalysts Will va ry considerably Vandcanbe determined only by previous vexperience with a particular catalystor byprelinimary experiments with the catalyst to' be" usedQfPrevious experience witlrthe particular-catalyst which is to be used willenalilelthe engineer to determine the amountsof catalyst requiredto give a series of varying pereentag,es of conversion, under adiabatic conditions and zit-the gas speed determined ,upon` of 4sulfur 'dioxidel gas of thev predetern'iined strength tobe used in thefsys f teni. If the information regarding p: vious operation is not available it will be necessary to run'anumber of prelnnmary experiments to deternnne the amounts of the particular catalyst which are required to give a series of ('liffercnt degrees of conversion of the gas to be handled in the system. These experiments may be conducted by placing 'a series of increasing' amounts of catalyst in an eX- perimental scale converter chamber, passing t through the chamber a stream of sulfur diox- "ide gas of a .strength equal to, andl at a gas speed equivalent to, that to be used in the commercial system, and determlnlng the percentage of sulfur dioxide converted'to sultur trioxide. In conducting such experiments it is, of course, essential to simulate plant conditions as nearly vas possible by maintaining i gas at about 400 C. and by insulating the converter chamber, Having obtained this data, either by previous experience or by experimental tests, a curve similar tor the curve O-A1A of 2 should then `be plotted, and-a point deter? mined upon this curve, where the rate of reaction, and` percentage conversion obtained perunit 'of catalyst, begins to rapidly fall off. This point will correspond to the point A of liig. 2. The amount of ,catalystl to be used in the first converter sta-'f ,lfe will then be the amount required to reach'thispoint.`

The desigmng engineer-.should then pro' vide for cooling the gasesto the proper extent, i. e. to about 400 C. positive, controllable cooling means, and then another curve should be plotted beginning at ther point corresponding to point A to determine the amount of catalyst required for the second stage. As is well known, the composition of the gas will be changed following the first con-verter stage, owing to the conversion of a certain amount of the sulfur dioxide and oxygen to sulfur trioxide, and

the new composition'may be readily determined by calculation based upon the percentage conversion which has taken place.

This new gas composition should,`of course,

'be'used in making use of prior data, or inv ruiming experiments, upon which to base the curve for the Isecond stage. In other respects the procedure for'plotting the curve for the second stage will be exactlyv the same as for the first stage. When handling a weak sulfur dioxide gas generating a relatively small amount of heat in the conversionfreaction it may be found that the curve for the second stage extends upwardly within the neighborhood of Sti-97% conversion, oftI .whatever overall conversion is desired in the system, without requiring an 'excessive amount'of catalyst. Under these conditionsthe system may be limited to a two stage system and the conversion carried substantially to equilibrium in the second and yfinal stage. yFor example, under most conditions when using a platinum catalyst a two stage system of operation will be found eiiicient.V for use with a gas containing up to within' the neighborhood of 7% sulphur dioxide. Above this strength of gas it will generally be found more economical in platinum to place sufficient catalyst in the second stage to bring the reaction only to the point where the rate of reaction begins to rapidly fall otl", such for instance as the point B1 of Fig. 2, and then to bring the reaction-,to substantial equilibrium in a third and final converter. IVith gas strengths above 12% four' stages may be found most economical. In 95 any case the proper number of stages which will be most economical in platinum for a given gas lstrength may be determined by plotting the curves for operation with variyous numbers of stages (in the manner described above for the first and second stages) and determining which arrangement will require the least amount of platinum for a given overall percentage conversion.

4 The system will be capable of continuous 105 operation with but little fluctuation. If by reason of changing trade conditions t it becomes necessary o change thestrength of gas being handled, the system will be found to be. quite iexible in its adaptability to strengths of gas differing from that strength for which the system was designed. For example, a. three-stage system designed particularly lfora 9% gas may be used with an 11% gas. Under these conditions the over- 115I all conversion will drop slightly, but by ade# quat-e removal of heat in the heat transferrers to take care of the additional heat generated in the conversion of the stronger gas, the total conversion will compare favorably with conversions in a plant where equilibrium is reached in every converter stage@ The system may then not be operating at maximum efficiency, but nevertheless a substantial saving in platinum will be effected 125 by reason of the employment of the principles of my invention. 1 l While I have illustrated my invention in connectiorrwith a three-stage converter system, it is of Course clear that the broader prineiplesof my invention may be applied irrespective of the number of stages. The essential conditions to be .observed in any case are to carryon the reaction in adiabatic stages, with positive intermediate cooling, to stop the reaction in all but the iinal convei'ter at that point where the reaction rate begins to fall 0E and then to carry the conversion to substantial equilibrium only in the final converter, and to place the proper quant-ity of platinum in each converter to obtain the latter result. lVlien operating a three-stage'system the relative proportions of platinum in the stages should be substantially'as indicated above. These proportions will vary, of course, with the number of stages.

By the term adiabatic as used in the specification and claims I mean to include any operation of the converter without a positive cooliigaction thereupon, that is,

v where precautions are taken to conserve the heat Within the converter, as by proper heat insulation. It is of course clear that even Witliheat insulation of the converters a certain amount of heat Will be lost by radiation and convection. By positive cooling I mean such cooling as is designed to accom- Aplisli the abstraction of a definite amount of heat from, or tlieattainment of a definite lowering 'in temperature of,l the hot gases in the system, as distinguished from cooling resulting merely from radiation and convection, or other means where the abstraction of heat from the system varies andis not controlled.

I have given, as an illustrative example,-

the application of my invention to a process in which la substantially quantitative yield has been obtained before absorption of the sulfur trioxide produced. Itis clear, however, that my invention is not limited to sucli an application. The reaction, for instance, may be carried on in stages with intermediate absorption of the sulfur trioxide produced. `In this case the first converters will be run below equilibrium conditions, and the conversion carried to y,substantial equilibrium only in the linal'onverter, in a manner similar to the first mentioned system.

I claim: I

l. The process of converting sulfur dioxide tov sulfur trioxide by. catalytic oxidation which consists in carrying on the converl' sion inl separate 'adiabatic stages, controlling the temperature of the gases entering the conversion stages by positive cooling, causiig the conversion reaction in all butthe final stage to proceed only to that point at which the reaction rate begins to rapidly fall ofl', and carrying the reaction to substantial equilibrium only in thefinal converter.

2. The improvement inthe process of converting sulfur dioxide to sulfur trioxide by catalytic oxidationin which the `reaction is carried on in separate stages with separate intermediate cooling of the partially converted gases to the-temperature desired for the next conversion, which consists in so proportioning the amounts of catalyst in the various stages as to obtain substantial equilibrium conversion only in the last section of catalyst material.

3. The process of converting sulfur dioxide to sulfur ti'ioxide by catalytic oxidation which consists in carrying on the conversion in separate stages, maintaining substantiallyl uniform temperature conditionsl throughout the catalyst at right angles to the direction of gas flow, controlling the temperature of the gases entering the conversion stages, causing the conversion reaction in all but the final stage to proceed only to that point at which the reaction rate begins to rapidly fall olf, and carrying the reaction to substantial equilibrium only in the final converter.

4. The process of converting sulfur di oxide to sulfur trioxide by lmeans of a platinum catalyst which consists in carrying on the conversion in three adiabatic stages, conftrolling the temperature of the gases entering the conversion stages, passing the gases in each stage in contact- Witli amounts of platinum constituting the following percentages of the whole, first stage 46%, second stage F95 10-1475, third stage 80-86% and providing su-Iiicient total platinum to bring about an overall conversion of approximately SH3-97%. l

5. The process of converting sulfur dioxide to sulfur trioxide by catalytic oxidation which consists in carrying on the conversion` of a 'gas containing 8`-12% S'O2 in three sub- 'stantially adiabatic stages, introducing the gas into the first stage at approximately 400 C.,'providing an amount of platinum in the firstconverter to obtain `'only a conversion of 842% below the equilibrium conversion,

cooling the gas to approximately 400 C. and

introducing the cooled gas into the second converter, providing an amount of platinum in the second converter to obtain only a conversion of 443% below equilibrium conversion, cooling the gas to approximately 400 C, and introducing the cooled gas into the third converter, and providing an amount of last stage, so as to cause the gas topass out 10.

platinum in the third converter to obtaina of Contact With the catalyst at about the time,A

final conversion of approximately 95-97%. when the rate of reaction begins to fall off 7. In the process of converting sulfur dioxsubstantially and then carrying the reaction 5 ide to sulfur trioxide by catalytic oxidation to substantial equilibrium only in the last in Which the reaction is carried on in stages', stage.

that improvement which comprises control- In testimony Wl1ereof,I alx my signature.

ling the temperature and proportioning the amount of catalyst in the stages, except thev JESSE G. MELEN DY. 

